This invention relates to apparatus for measuring fluid flow and more particularly to apparatus for providing a density compensation to measured fluid flow.
Various devices have been developed for enabling determination of volumetric fluid flows and in many instances, the particular fluid flow to be measured will not be altered appreciably under different ambient conditions. For example, water meters may be simply calibrated to measure fluid flow and subsequent recalibration is generally unnecessary as the density of water is hardly affected by the mere retention thereof under varying ambient temperatures. However, for numerous other fluids such as pressurized gases and cryogenic liquids, which must be retained or transported in appropriately pressurized or insulated vessels, fluid density will vary substantially as a consequence of unavoidable pressure and heat leaks attendant to such vessels. Such leaks will inevitably occur during the transport of a pressurized gas or cryogenic liquid from, for example, a production site to a point of end use. The term "pressurized" gas is generally defined as any gas retained under a pressure greater than atmospheric while the term "cryogenic" liquid refers to those substances existing in a liquid phase at temperatures below, for example, -150.degree. F. Exemplary of such cryogenic liquids is liquid nitrogen which is commonly introduced into transport vessels at a temperature of 77.degree. K (-320.degree. F) and at atmospheric pressure. Alternately, the cryogenic liquid may comprise liquid oxygen or liquid argon. In addition, other materials such as liquid carbon dioxide, are also transported in pressurized containers. However, as heat leaks into such vessel are inevitable, the resulting temperature of the transported cryogenic liquid is inherently higher than the temperature at which such liquid was introduced into the vessel.
Generally, in the sale and distribution of industrial pressurized gases and cryogenic liquids, such materials are introduced into a transport vehicle such as a trailer at a production site. The amount of material introduced into such trailers is usually measured at standard temperature and pressure conditions, e.g. liquid nitrogen is generally supplied at a temperature of 77.degree. K, 0 p.s.i.g., as noted above, and the mass or weight of such introduced material may be readily determined from a volumetric flow meter calibrated to such standard conditions. However, during transport liquid nitrogen may easily undergo a temperature increase from 77.degree. to 90.degree. K which typically results in a density reduction of approximately 8% although larger density reductions will be encountered for greater increases in temperatures. Consequently, the weight of each volumetric unit of liquid N.sub.2 discharged from a trailer or the like will represent 92% of the weight of each volumetric unit, as measured by a volumetric flowmeter, introduced into the trailer. Therefore, by charging or billing a customer based on the volumetric flow of discharged liquid, the customer will be overcharged as a consequence of actually receiving a lower amount (weight) of liquid than would be indicated by an uncompensated volumetric reading of the discharged liquid flow. Similar changes in density and discrepancies between the introduced mass and discharged mass of pressurized gases will also occur as pressure leaks will contribute to a density reduction in each volume unit of the discharged gas. Thus, in the case of cryogenic liquid and pressurized gases, the density of which is sensitive to temperature and pressure leaks, respectively as aforesaid, changes in a parameter peculiar to such fluids will have significant effects on the density of such fluid upon discharge thereof after the fluid is subjected to such leaks. This parameter may be defined as the dominant parameter affecting the density of the particular fluid. In the case of pressurized gases this parameter is pressure and the dominant parameter affecting the density of cryogenic liquids is temperature.
One technique which has been proposed for solving the persistent problem of obtaining inaccurate volumetric flow measurements of discharged pressurized gases and cryogenic liquids is the recalibration of a volumetric flow meter immediately prior to the passage of a particular fluid therethrough. However, in practice such recalibration is time consuming and requires delicate adjustments, and as in many instances the operator of a storage facility or transport vehicle possesses limited skill in such areas, the concept of meter recalibration has not been proven effective in practice. Consequently, it is common practice for an operator to merely estimate the density of discharged fluid by applying a nominal correction factor to the actual reading appearing on a volumetric flowmeter of, for example, a cryogenic liquid trailer. Clearly, this technique fails to accurately provide density compensations to the measured volumetric fluid flow and thus fails to indicate with precision the actual amount or mass of fluid which should be charged or billed to a customer receiving the same. Therefore, it has been found that a clear need exists for obtaining simple and accurate density compensated measurements of pressurized gas and cryogenic liquid flows.